1. Field of the Invention
The present invention relates to image sensors. More particularly, the invention relates to shared-pixel-type image sensors for controlling the capacitance of a floating diffusion region.
2. Description of the Related Art
An image sensor converts optical images into electronic signals. With advancements made in computers and communication devices, demand for image sensors having improved performance characteristics has increased in various fields, e.g. digital cameras, camcorders, personal communication systems (PCSs), game machines, security cameras, medical micro cameras, robots, or the like.
A MOS image sensor may have a simple driving method and may be implemented by various scanning methods. In MOS image sensors, a signal processing circuit may be integrated in one chip, and thus a size of a product including such a MOS image sensor can be reduced. Further, MOS image sensors are generally compatible with MOS process technology, and thus the manufacturing cost can be reduced. Furthermore, because a MOS image sensor may consume a relatively low amount of power, MOS image sensors may be easily employed in products requiring a restricted battery capacity. MOS image sensors may also operate at relatively high resolutions. Accordingly, MOS image sensors are being used in various technologies.
However, as pixel integration is increasing in order to achieve increased resolution, areas occupied by photoelectric conversion elements are being reduced. Such products having photoelectric conversion elements occupying smaller areas may have lower sensitivity and a saturation signal amount.
Accordingly, an active pixel sensor array with a plurality of photoelectric conversion elements sharing a reading element has been proposed. By sharing a reading element, an area of the photoelectric conversion element serving as a light receiving unit is increased so as to increase light receiving efficiency.
Meanwhile, when a fill factor is increased by increasing an area of a photoelectric conversion element serving as a light receiving unit, an area of a floating diffusion region is reduced. If the relative area of the floating diffusion region is decreased, capacitance of the diffusion region is decreased.
To improve sensitivity, it should be possible to quickly accumulate a large amount of charge in the floating diffusion region in order to improve the sensitivity. Also, capacitance of the floating diffusion region should be increased in order to accumulate the large amount of charge in the floating diffusion region.
Meanwhile, an increase in capacitance of the floating diffusion region may cause a dynamic range to be reduced. Therefore, in order to satisfy such characteristics, structures for sensors enabling capacitance of the diffusion region to be appropriately controlled are desired.